1. Field of the Invention
The present invention relates to a multilayered ceramic substrate production method, and more particularly, to a production method for a multilayered ceramic substrate having a cavity for mounting and holding an electronic component therein.
2. Description of the Related Art
In recent years, there has been an increasing demand for smaller and lighter electronic components with more functions, higher reliability, and the like. Consequently, there has been a need to improve the technique for mounting components on a substrate. The most typical and effective method for improving the mounting technique on the substrate is to increase the density of wiring on substrates.
In order to respond to such a desired increase in density of wiring of the substrates, multilayered ceramic substrates produced by stacking a plurality of green ceramic sheets having conductive films and the like printed thereon, and by pressing and firing the green ceramic sheets, have been developed.
In order to reduce the size and thickness of the multilayered ceramic substrate itself, it is effective to form, in the multilayered ceramic substrate, a cavity for mounting an electronic component therein.
In such a multilayered ceramic substrate having a cavity, however, the flatness of the bottom surface of the cavity is often degraded as a result of a firing step conducted during the process of obtaining the multilayered ceramic substrate. This tendency is marked particularly when the cavity has a step portion therein. As the number of such step portions increases, the problem worsens.
A so-called shrinkage-reducing process is frequently adopted in the firing step for producing the multilayered ceramic substrate. That is, the firing step in the multilayered ceramic substrate production method using the shrinkage-reducing process is performed in a state in which shrinkage-reducing layers containing an inorganic powder material, which is not fired at the firing temperature of a ceramic material contained in the plurality of green ceramic sheets, are placed so as to cover both end faces in the sheet-stacking direction of a green sheet stack formed by stacking the green ceramic sheets. In such a shrinkage-reducing process, the shrinkage-reducing layers are not substantially shrunk in the principal surface direction during the firing step, and the green ceramic sheets are thereby restrained. Therefore, the obtained multilayered ceramic substrate is not prone to nonuniform deformation. This makes it possible to reduce undesirable deformation and distortion in wiring conductors disposed in connection with the multilayered ceramic substrate, and to thereby advantageously increase the density of wiring conductors.
In the multilayered ceramic substrate having a cavity, however, it is difficult to exert the restraining action of the shrinkage-reducing layer onto the bottom surface of the cavity, and the above-described problem of degradation in flatness of the bottom surface of the cavity is often not overcome by simply adopting the shrinkage-reducing process. Moreover, the degradation in flatness of the bottom surface of the cavity is more considerable when undesirable deformation of the portion of the multilayered ceramic substrate other than the cavity is inhibited.
In order to overcome the above problems, for example, Japanese Unexamined Patent Application Publication No. 5-167253 or No. 11-177238 discloses that a firing step is performed while the interior of a cavity is filled with an inorganic material powder, which is not fired at the firing temperature of a ceramic material contained in green ceramic sheets or with a paste containing the inorganic material powder. In this method, however, a difference in shrinking behavior in the sheet-stacking direction is made between the green ceramic sheets and the inorganic material powder in the cavity during firing. For this reason, the periphery of the cavity is prone to cracking. This problem is more serious particularly when the cavity has a step portion therein, and as the number of such step portions increases.
Accordingly, it is an object of the present invention to provide a production method for a multilayered ceramic substrate having a cavity which can overcome the above-described problems.
The present invention relates to a multilayered ceramic substrate production method including the steps of: preparing a first green ceramic sheet having an opening for forming a cavity, and a second green ceramic sheet having no opening at least at a position corresponding to the opening; producing a green sheet stack having a cavity defined by the opening by stacking the first green ceramic sheet and the second green ceramic sheet so that an aperture of the cavity is placed on at least one end face in the sheet-stacking direction of the green sheet stack; pressing the green sheet stack in the sheet-stacking direction; and firing the green sheet stack at a temperature at which a ceramic material contained in the first and second green ceramic sheets is fired. In order to solve the above technical problems, the multilayered ceramic substrate production method of the present invention includes the following structures.
The multilayered ceramic substrate of the present invention further includes a step of preparing a shrinkage-reducing sheet containing an inorganic material powder which is not fired in the step of firing the green sheet stack.
In the step of producing the green sheet stack, the shrinkage-reducing sheet is placed so as to close the aperture of the cavity and to cover the end face in the sheet-stacking direction of the green sheet stack.
The step of pressing the green sheet stack is performed so that a pressing force is exerted onto a bottom surface of the cavity, so that the shrinkage-reducing sheet is cut along the rim which defines the aperture of the cavity, and so that a shrinkage-reducing sheet piece formed of a part of the shrinkage-reducing sheet is placed on the bottom surface of the cavity.
As a result, the step of firing the green sheet stack is performed in a state in which the shrinkage-reducing sheet piece is placed on the bottom surface of the cavity.
Preferably, the shrinkage-reducing sheet piece is removed after the green sheet stack is fired.
Preferably, the step of pressing the green sheet stack is performed so that the green sheet stack is pressed via an elastic member placed outside the shrinkage-reducing sheet.
Preferably, a rigid plate having an opening of a size substantially equal to or slightly smaller than that of the aperture of the cavity is prepared, and the step of pressing the green sheet stack is performed in a state in which the rigid plate is placed between the elastic member and the green sheet stack. The rigid plate may be placed between the shrinkage-reducing sheet and the elastic member, or between the green sheet stack and the shrinkage-reducing sheet.
Preferably, the shrinkage-reducing sheet is placed in contact with the end face in the sheet-stacking direction of the green sheet stack in the step of producing the green sheet stack, and the step of firing the green sheet stack is performed in a state in which a portion of the shrinkage reducing sheet remaining after the shrinkage-reducing sheet piece is removed from the shrinkage-reducing sheet remains on the end face in the sheet-stacking direction of the green sheet stack.
Preferably, when the cavity is formed on only one end face in the sheet-stacking direction of the green sheet stack, a second inorganic material powder, which is not fired during the step of firing the green sheet stack, is prepared, and a shrinkage-reducing layer containing the second inorganic material powder is placed so as to cover an end face of the green sheet stack opposite from the end face with the cavity when the green sheet stack is produced.
A second inorganic material powder which is not fired in the step of firing the green sheet stack may be prepared, and a shrinkage-reducing layer containing the second inorganic material powder and having a portion from which the aperture of the cavity is exposed, may be placed so as to cover the end face in the sheet-stacking direction of the green sheet stack in the step of producing the green sheet stack.
Preferably, the second inorganic material powder is the same as the inorganic material powder contained in the shrinkage-reducing sheet.
Preferably, the shrinkage-reducing layer is removed after the green sheet stack is fired.
The present invention is advantageously applied particularly to a case in which the cavity has therein at least one step portion which defines an aperture smaller than the above aperture. In this case, the shrinkage-reducing sheet is also cut along the rim of the step portion as a result of the step of pressing the green sheet stack, and a shrinkage-reducing sheet piece is also placed on a bottom surface formed on the step portion.
The shrinkage-reducing sheet may be subjected beforehand to processing which allows the shrinkage-reducing sheet to be easily cut at a specific portion. Preferably, perforations are formed beforehand in a specific portion of the shrinkage-reducing sheet.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.